Conventionally, electrical submersible pump (“ESP”) assemblies have been made up of a series of interconnectable modular sections including one or more pump sections with an associated fluid intake, a motor section and a seal section. Each of these sections includes an outer housing and a central drive shaft. At present, the shafts and their connections are designed so that they can primarily transmit a compression load. The shaft of each section is interconnected with the shaft of the adjacent section by straight splines for transmitting torque only. There are normally no securing members used that would resist pulling apart of the shafts. Placement of the shafts or shaft connections under tension loads will cause the connection to separate easily.
This sort of “compression only” connection between shaft members is permissible when a standard ESP configuration is used wherein the pump section(s) are located above the seal and motor sections. In pump assemblies with pumps mounted above the motor, down thrust on the shafts typically passes downward to a thrust bearing located in the seal section between the pump or pumps and the motor. The motor also normally has a thrust bearing. The thrust bearings in the seal and motor sections support the pump sections, and the shaft members in the pump sections are not placed in tension.
When a “bottom intake” ESP configuration is used, however, the pump section(s) are located below the motor and seal section in the wellbore. An expensive thrust bearing is normally mounted to the lower end of the pump section(s) to support the ESP components. Downward force on the shaft sections in the pump(s) passes to the thrust bearing at the lower end of the pump(s).
Although it might be possible to simply pin or weld the shafts of adjacent ESP sections together to transmit tension, there is an operational problem with doing so. In practice, it is difficult to assemble and disassemble the pinned shafts since they reside within the housings. If the connection is welded, it is quite difficult to disassemble the sections after removal of the pump assembly from the wellbore.